Field focusing and mapping in an electrode array

ABSTRACT

The present invention is a system for mapping a high resolution image to a lower resolution electrode array and, by applying varying stimulus to neighboring electrodes, creating a perceived image greater in resolution than the electrode array. The invention is applicable to a wide range of neural stimulation devices including artificial vision and artificial hearing. By applying a sub-threshold stimulus to two neighboring electrodes where the sum of the stimuli is above the threshold of perception, a perception is created in neural tissue between the two electrodes. By adjusting the stimulus on neighboring electrodes, the location of stimulation can be altered. Further, noise can be applied to the stimulating electrode or its neighboring electrodes to reduce the threshold of stimulation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. Provisional Application60/400,018, filed Jul. 30, 2002, “Entitled Current Steering to ImproveResolution in an Electrode Array.”

GOVERNMENT RIGHTS NOTICE

This invention was made with government support under grant No.R24EY12893-01, awarded by the National Institutes of Health. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention is generally directed to electrode arrays, andmore particularly to implantable electrode arrays for medical devices.

BACKGROUND OF THE INVENTION

Arrays of electrodes for neural stimulation are commonly used for avariety of purposes. Some examples include: U.S. Pat. No. 3,699,970 toBrindley and “The Sensations Produced by Electrical Stimulation of theVisual Cortex” by G. Brindley and W. Lewin, J. Physiol (London)196:479-493:1968. Brindley's paper and patent describe an array ofcortical electrodes for visual stimulation. One cortical electrode isused for each light percept. Each electrode is attached to a separateinductive coil for signal and power. U.S. Pat. No. 4,573,481 to Bullaradescribes a helical electrode to be wrapped around an individual nervefiber. U.S. Pat. No. 4,628,933 to Michelson describes an electrode arrayfor retinal stimulation. U.S. Pat. No. 4,837,049 to Byers describesspike electrodes for neural stimulation. Each spike electrode piercesneural tissue for better electrical contact. U.S. Pat. No. 5,215,088 toNorman describes an array of spike electrodes for cortical stimulation.Each spike pierces cortical tissue for better electrical contact. U.S.Pat. No. 5,109,844 to de Juan describes a flat electrode array placedagainst the retina for visual stimulation. U.S. Pat. No. 5,935,155 toHumayun describes a retinal prosthesis for use with the flat retinalarray described in de Juan.

In addition to the electrode arrays described above, there are severalmethods of mapping a high resolution camera image to a lower resolutionelectrode array. U.S. Pat. No. 6,400,989 to Eckmiller describesspatio-temporal filters for controlling patterns of stimulation in anarray of electrodes. The assignee of the present applications has threerelated U.S. patent application Ser. No. 09/515,373, filed Feb. 29,2000, entitled Retinal Color Prosthesis for Color Sight Restoration;Ser. No. 09/851,268, filed May 7, 2001, entitled Method, Apparatus andSystem for Improved Electronic Acuity and Perceived Resolution Using EyeJitter Like Motion; filed on current date herewith, entitled UserDirected Pixel Re-Mapping. All three applications are incorporatedherein by reference.

The density of neural tissue is far greater than currently availableelectrodes arrays. Using artificial vision as an example, the humanretina has approximately 4,000,000 receptors. Further those receptorsare not evenly distributed, but are far denser near the fovea, at thecenter of the retina. The spacing of receptors near the fovea isapproximately 5 μm. The best know technology for producing electrodescapable of stimulating retinal neurons requires 40 μm electrodes with 20μm spaces. Other neural tissues, such as cortical tissue, is about thesame scale as retinal tissue. Other neural tissue, such as that found inthe optic nerve is far denser. Obtaining a high resolution image issimple and inexpensive using charge coupled device (CCD) cameras. Whilethe mapping systems described above help present the most relevant datagiven the limited resolution of the electrode array, they do notincrease the perceived resolution for the user of a visual prosthesis. Amethod is needed to direct a high resolution image to a lower resolutionelectrode array while achieving the highest possible perceivedresolution to the individual stimulated by the electrode array.

It is further advantageous to reduce the power needed to stimulate aneuron. As stated above, the smallest possible electrode spacing withcurrent technology is 40 μm electrodes with 20 μm spaces. This assumes aminimal charge requirement. The charge needed varies from retinal toretina, varies across the surface of the retina, and varies over time asretinal disease progresses. Charge density is the charge transferred byan electrode, divided by the surface area of the electrode. As chargedensity increases, electrochemical reactions at the electrode surfacebecome more intense leading to dissolution of the electrode. It istherefore, advantageous to stimulate neurons with the minimum chargenecessary.

SUMMARY OF THE INVENTION

The present invention is a system for mapping a high resolution image toa lower resolution electrode array and, by applying varying stimulus toneighboring electrodes, creating a perceived image greater in resolutionthan the electrode array. The invention is applicable to a wide range ofneural stimulation devices including artificial vision and artificialhearing. By applying a sub-threshold stimulus to two neighboringelectrodes where the sum of the stimuli is above the threshold ofperception, a perception is created in neural tissue between the twoelectrodes. By adjusting the stimulus on neighboring electrodes, thelocation of stimulation can be altered. Further, noise can be applied tothe stimulating electrode or its neighboring electrodes to reduce thethreshold of stimulation.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will be best understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the prior art method of mapping electrodes.

FIG. 2 depicts mapping electrodes according to the present invention.

FIG. 3 is a voltage over time plot of stimulating pulses according tothe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims.

FIG. 1 shows the prior art. Using a visual prosthesis as an example, a 6by 4 array of image pixels 10 is mapped to two electrodes, electrode A12 and electrode C 14 which may be placed on the retinal surface orplaced within the visual cortex. The left 12 pixels (columns 1-3) aremapped to electrode A 12 and the right 12 pixels (columns 4-6) aremapped to electrode C 14. The most common mapping is simply taking anaverage of the twelve pixels. Various modifications to a simple averageare known which highlight edges, increase contrast, or otherwise makethe limited information more relevant.

All neurons have a threshold potential that causes that neuron to “fire”and send a signal along its neural pathway. It is believed that thefiring is caused by creating a capacitive field around the neuron.Although the exact mechanism is not well understood, applying anelectrical current at a certain level (super-threshold) will cause theneuron to fire. Current below that level (sub-threshold), will have noeffect on the neuron. Applicant has determined through experimental usethat applying a sub-threshold electrical current to two adjacentelectrodes, where the sum of the signals applied to the two electrodesis super-threshold, will cause a neuron to fire between the two adjacentelectrodes. The location between the electrodes is dependent on relativecurrent. A higher current will steer the location of stimulation towardthe electrode with the higher current. For simplicity, the effect isdescribed in a single dimension, but works in two dimensions, such as onthe retinal surface, or in three dimensions, such as in cortical tissue.

By using this effect, it is possible to remap image pixels in the visualprosthesis described above to utilize the neurons between electrodes asshown in FIG. 2. The left 8 pixels (columns 1 and 2) are mapped toelectrode A 12, the right 8 pixels (columns 5 and 6) are mapped toelectrode C 14, and the center 8 pixels (columns 3 and 4) are mapped tothe space B 16 between electrode A12 and electrode C 14.

It is possible to enhance this effect, reduce power consumption andreduce the chance of damage to neural tissue, by balancing the currentsapplied on adjacent electrodes. As an example, if the threshold forneural stimulation is 1 μA (micro amp), and 0.5 μA is applied toelectrode A 12 and −0.5 μA is applied to electrode B 14, then one ormore neurons will fire causing the perception of a pixel a point B 16,halfway between electrode A 12 and electrode C 14. Applying a charge of0.8 μA to electrode A 12 and −0.2 μA to electrode C 14 will steer thepoint of perception toward electrode A 12.

It is possible to obtain multiple point stimulations by rapidly changingthe stimulation. Just as we perceived the individual frames of movie asfluid motion, we perceive rapid repeated stimulation as a single image.An example is shown if FIG. 3.

FIG. 3 shows the charge applied to electrodes A 12 and C 14 over atypical display cycle. Pulse 18 applied to electrode A 12 is the averageof pixels in column 1 and 2. Pulse 20 is equal and opposite to balancethe charge. Pulse 22 is one half of the average of the pixels in columns3 and 4. Pulse 24, applied to electrode C 14 is equal and opposite topulse 22 (the other half of the average of the pixels in columns 3 and4). Since it is helpful to balance the charge on each individualelectrode, pulse 26 is equal and opposite to pulse 22, and pulse 28 isequal and opposite to pulse 24. Finally, pulse 30, applied to electrodeC 14 is equal to the average of the pixels in columns 5 and 6. Pulse 32is equal and opposite to pulse 30. The decay period in the human visualsystem is about 1/50 of a second. Hence movies displayed at 60 framesper second look fluid while older movies displayed at 30 frames persecond appear to flicker. Hence the time line shown in FIG. 3 must beshorter than 1/50 of a second to achieve the perception of a continuousimage. While FIG. 3 shows stimulation of a single point halfway betweenelectrode A 12 and electrode C 14, it should be understood that theprocess can be repeated to multiple times to stimulate multiplelocations between electrode A 12 and electrode C14.

Applying a noise signal rather than a straight DC charge can reduce thethreshold of perception and further improve the process. The noisesignal can be applied to the stimulating electrode where a percept isapplied directly at the electrode or on neighboring electrodes. Noisestimulation should be sub-threshold and can be immediately beforestimulation of a percept or, when on a neighboring electrode,simultaneously with stimulation of a percept.

Accordingly, what has been shown is an improved method of stimulatingneural tissue for increased resolution. While the invention has beendescribed by means of specific embodiments and applications thereof, itis understood that numerous modifications and variations could be madethereto by those skilled in the art without departing from the spiritand scope of the invention. It is therefore to be understood that withinthe scope of the claims, the invention may be practiced otherwise thanas specifically described herein.

1. A method of stimulating neural tissue comprising: providing at leasttwo electrodes suitable for contact with the neural tissue comprisingneurons, each neuron within the neural tissue having a predeterminedindividual threshold of firing; and energizing each of selected twoelectrodes, selected from said at least two electrodes, at a level lessthan the predetermined individual threshold of firing of a neurondisposed directly adjacent either of the selected two electrodes,wherein the combined signal from said selected two electrodes is greaterthan the threshold of firing of a neuron between the selected twoelectrodes causing a percept different from a percept caused by a neurondisposed directly adjacent either of the two selected electrodes.
 2. Themethod according to claim 1, wherein said step of energizing is repeatedat different levels suitable to stimulate different neurons.
 3. Themethod according to claim 2, wherein said step of energizing is repeatedrapidly and wherein signals from different neurons form the perceptionof a single sensation.
 4. The method according to claim 1, wherein thestep of energizing includes energizing with opposite polarities.
 5. Themethod according to claim 4, wherein the sum of said opposite polaritiesis zero.
 6. The method according to claim 1, wherein the step ofenergizing includes energizing at least one of the two electrodes withopposite polarities.
 7. The method of claim 1, wherein the neuron iswithin a subject's retina.